Bone adapts to the mechanical loads it experiences. This adaptive response takes place by bone modeling and remodeling, and is driven by the mechanical stimuli experienced by the bone. The degree to which bone adapts depends on the degree to which the mechanical stimulus deviates from some threshold stimulus values. This ability of bone to adapt to externally applied mechanical stimulus has allowed exercise-based interventions as a practical option to prevent bone loss and enhance bone strength. However, determining the mechanical stimulus thresholds required to initiate adaptive response is important to the success of such interventions.

The magnitude of the adaptive response has been attributed to the characteristics of the mechanical stimulus in animals. However, the contribution of the different mechanical stimulus characteristics to the adaptive process and the mechanical stimuli thresholds above which bone adaptation is initiated are currently unknown in humans. The objective of this research was to understand the quantitative relationship of human bone to its mechanical environment, with the long term goal of designing and evaluating exercise interventions to prevent or slow bone loss that can lead to osteoporosis, and improve fracture strength.

A novel in-vivo wrist loading model was used to accomplish the objectives of this research. Methods for subject specific finite element model generation to predict the surface strains at the distal radius were validated with high accuracy (r=0.968, RMSE=11.1%), and were used to assess loading-induced bone strain in the subjects. An increase (or the prevention of a decrease) in ultra-distal radius size and mass was the primary adaptation response to the axial compression of the radius, and this response was more directly related to strain magnitude than observed between changes in bone mineral density and the mechanical measures of the applied loads at the local level within the bone. Although a strain-dependant site-specific behavior has previously been shown in animals, to our knowledge, this was the first time that the localized adaptation behavior of bone was tested in humans.

In summary, we have developed an in vivo loading model of the human radius for the purpose of understanding influence of mechanical environment on bone adaptation. The loading task was capable of producing an osteogenic response, and along with the validated in-vivo FE model, we were able to test the relation between the mechanical characteristics of the applied loads and the resultant changes in the bone mineral parameters. In addition to its usefulness for exploring bone adaptation in humans, this research also acts as a step towards designing effective targeted mechanical interventions to increase (or prevent the decrease of) bone strength.